Important characteristics of paper used in printing applications are the "lay flat" and "cockling" properties of the paper. Any wave or undulation in paper can result in poor print quality. Such undulations can arise, for example, when paper is wetted or exposed to humidity. This behavior is generally more pronounced when the paper undergoes humidity cycling which can cause the edges of the paper to curl inward resulting in surface height variations. The localized surface variations or buckling is known as "cockling" and originates from built in strains that are released and cause the local expansion or contraction of the paper. The more macroscopic curl behavior originates in the two-sidedness of the sheet and depends on time as well as humidity.
Depositing ink on paper can cause or contribute to more pronounced cockling. This is referred to as "wet cockle". As the ink dries on the paper the undulations diminish and the resulting deflections in the paper as compared to a flat sheet are referred to as "dry cockle". The severity of both wet and dry cockle is largely paper dependent, more specifically dependent on such variables as fiber furnish, basis weight and formation.
Wet cockle can be a severe problem in ink jet printing applications since wet cockle may lead to "head crash", where the print head contacts the paper as it travels up and back during printing, resulting in smearing, paper jams, and damage to the printing machine. Dry cockle and curl can adversely affect the appearance of the paper for the end user.